In the marketplace, there is bad fuel with a high concentration of sulfur. If it is detected that such fuel has been supplied into the fuel tank, the fuel can be removed therefrom in case of need. The exhaust gas of the fuel with a high concentration of sulfur includes a relatively large amount of SOX. SOX temporarily deteriorates a three way catalyst. Accordingly, it is suggested that an O2 storage ability of a three way catalyst device before and after the fuel supply is estimated on the basis of the outputs of an O2 sensor arranged downstream of the three way catalyst device. It is determined that the fuel with a high concentration of sulfur has been supplied when the O2 storage ability has dropped to some degree (for example, refer to Japanese Unexamined Patent Publication Nos. 2003-148137, 2001-74727, 2005-344619, and 2000-320322).